Wouldn't the hens size need to be 'scaled' to the size increase of the rooster? A 9 - 10 lb rooster on a 4 - 5 (or 6) lb hen sounds like a bummer for the hen. He could break her, or seriously hurt her, couldn't he?
Actually you're right. My LF Cochin boys tore up my Wyandotte girls backs.
Either way, I think it would be pretty hard to make these birds a lot bigger, without breeding in a different breed. I'll be sticking with what I have, and ignore the rest of this discussion.